Screen printing apparatus for printing of viscous material

ABSTRACT

An apparatus for printing a viscous material onto a surface of a board through the medium of a screen having openings therein. The apparatus comprises a viscous material dispenser (S) of the type having a box-like housing (A) in which a chamber (5) for containing the material and a slit-like bottom aperture (3) communicating therewith are provided. A pair of blades (2A, 2B) depend from the housing (A) on opposite sides of the bottom aperture (3). A rear wall (8) of the housing (A) comprises a fixed upper wall section (8A) and a lower flap wall section (8B) swingably and slidably connected thereto and depending therefrom. The lower flap wall section (8B) defines the rear side of the bottom aperture (3). One of said blades which serves as a squeegee blade is fixed to the flap wall section. The flap wall section (8B) is connected to a pair of manually operable height-adjustable devices (22) respectively mounted on said housing and for adjusting the height of the flap wall section (8B) having the squeegee blade (2B) so that by adjusting said height by means of said devices the contacting pressure of the lower end of said blade (2B) to be applied against the upper surface of said screen (1) can be adjusted as desired. Also, the flap wall section (8B) is operatively engaged with a pair of actuating means (B) respectively mounted on said housing and for closing said aperture (8) when the dispenser is in a non-printing position and for opening it to supply a predetermined amount of the material contained in said chamber (5) onto the screen (1) through said aperture when the dispenser is in a printing position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an apparatus for dispensing aviscous material onto a surface through a screen having a plurality ofopenings therein, and more particularly to a screen printing apparatusfor printing solder paste onto a surface of a printed circuit boardthrough the screen that is used in producing surface mount technologycircuit boards suitable for use in high technology products.

2. Description of the Prior Art

A screen printing apparatus is known which has a spatula-like flexiblesqueegee which is arranged to be displaced back and forth along a pairof guide rails and is arranged to be raised and lowered relative to theguide rails. This printing apparatus is adapted to be in pressurecontact with an upper surface of the screen when it is lowered andplaced into a lower position where solder paste is printed onto asurface of a printed circuit board through the medium of the screen.This type of screen printing apparatus is disclosed in the JapanesePatent Gazette No. 1-55597 published on 27th Nov., 1989. In this type ofapparatus, a considerable amount of solder paste is manually suppliedonto the screen prior to operating the apparatus and the solder pastesupplied onto the screen in such a way is printed on the surface of thecircuit board by reciprocating the squeegee, while its lower end ispressed against the screen to introduce the solder paste into openingsof the screen.

Major disadvantages of this type of screen printing apparatus are thatas a considerable amount of solder paste placed on the screen is keptexposed to the air until the apparatus is put into operation, it tendsto be solidified due to evaporation of flux contained therein and/or tobe oxidized as it contains oxidizable components, such as lead. As theprinting of solder paste is performed by reciprocating the squeegee, thesolder paste tends to be excessively printed on a surface of the circuitboard. As a result, printed films or deposits tend to be blurred on thesurface of the circuit board and, in addition, a uniform printing effectcannot be obtained in every printing cycle. It is difficult to preventthe solder paste from acquiring of such changes as solidification,oxidation, etc., even if it is frequently kneaded on the screen withmanual labour during the period when the apparatus is not in operation.To use such a solidified and/or oxidized solder paste brings aboutundesirable printing effect. For this reason, it must be removed fromthe screen and replaced with new solder paste. This means that aconsiderable amount of solder paste becomes a useless waste to beabandoned. In addition, replacement of solder paste not only requiresthe manual labour, but also requires printing operation be stopped for acertain period of time. This results in an economic loss and workerexposure to lead which is a health concern.

Another screen printing apparatus is known which comprises a viscousmaterial automatic supply dispenser of the type having a housing of aclosed box type in which a chamber for containing the material and aslit-like bottom aperture communicating therewith, having a leading sidedefined by a front wall and a trailing side defined by a rear wall andbeing arranged to be normally closed and to be kept open toautomatically supply a predetermined amount of the material onto asurface of a board during printing. This type of screen printingapparatus is disclosed in U.S. Pat. No. 2,027,102 and in the Japanesepatent application No. H2-419135 published in 1992. In this type ofknown apparatus, the dispenser is provided with a flexible or pliablerear wall which serves as a squeegee to force the material supplied bythe dispenser onto an upper surface of a screen into openings in thescreen. Another screen printing apparatus which is similar to this typeof known apparatus and in which a slit-like bottom aperture in a housingis not arranged to be closed and opened is also known. This type ofapparatus is disclosed in U.S. Pat. No. 4,622,239. The apparatusdisclosed in this patent is provided with a pair of flexible blades assqueegee blades which are secured to, and depend from a housing onopposite sides of the bottom aperture to contact a screen.

A common feature of these known apparatuses of the type having such aviscous material automatic supply dispenser is that one or two squeegeesemployed by the dispenser are formed of a urethane rubber or the like.This feature tends to bring about disadvantages when the dispenser isused for screen printing of solder paste onto a surface of a printedcircuit board, as will be described later.

Another common feature of these known apparatuses of the type mentionedabove is that the dispenser lacks means for adjusting the contactingpressure of a squeegee to be applied against the upper surface of thescreen. Lack of such means also brings about disadvantages, as will alsobe described.

FIG. 13(a) shows the state in which a flexible squeegee N employed by aconventional screen printing apparatus is in pressure contact with ametal screen 1. As shown, the lower end of the squeegee projectspartially by deformation into an opening 1A in the screen. As a result,as shown in FIG. 13(b), printed solder paste 4 on a metalized area of aprinted circuit board P is deformed into a concave, so that uniformityin the printed paste thickness cannot be obtained.

When printing solder paste 4 which normally has a high viscosity withthe flexible squeegee N, the squeegee will experience high shear forces,causing the paste to separate into the constituent components (e.g.,solder particles and a suspending viscous flux) and a thin film of fluxis formed on the screen 1. The flux film left on the screen makesresidue of the paste which is detrimental to high quality printing ofsolder paste onto the printed circuit board P.

The amount of projection of the flexible squeegee N into an opening 1Ais larger as the area of each opening is larger, preventing high qualityprinting.

In the screen printing, printing is repeatedly performed, so that whenthe contacting pressure of squeegee applied against a screen is toostrong, the screen is caused to be damaged by abrasion. If the viscousmaterial to be printed is solder paste containing solder particles,abrasive friction between the screen and the squeegee will be increased,because the particles are forced to be pushed ahead under pressure.Conversely, if and when said contacting pressure is weak, then some ofthe flux and some of the solder particles will pass under the squeegeeand will be left as solder paste residue. In addition it will beimpossible for the squeegee to introduce a sufficient amount of solderpaste into each of the openings in the screen, preventing properprinting. Moreover, the printed solder paste, when depleted of flux,tends to remain in the screen openings and causes the problem ofclogging of the openings. It is impossible for the conventionaldispensers to adjust the contacting pressure of the squeegee orsqueegees to be applied against the screen as occasion demands.

SUMMARY AND OBJECTS OF THE INVENTION

The present invention is intended to eliminate the disadvantages ordrawbacks indicated with the prior art as described above and to providean improved apparatus for printing a viscous material, such as solderpaste onto a surface of a board, such as a printed circuit board throughthe medium of a screen having openings therein which is capable ofperforming a precise and high quality screen printing and which isparticularly suited for use in mounting surface-mounted components, suchas semi-conductor chips, resistors and capacitors etc., on the printedcircuit boards.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a rear view of one embodiment of the present invention;

FIG. 2 is a side view of the embodiment, showing the state in which thedispenser is performing a screen printing operation;

FIG. 3 is a side view of the embodiment, showing the state in which thedispenser is raised after it has finished screen printing;

FIG. 4 is a perspective view of the dispenser;

FIG. 5 is a perspective view of a rear wall of the dispenser;

FIG. 6 is an exploded perspective view of a part of the rear wall;

FIG. 7 is an exploded perspective view of the rear wall;

FIG. 8 is an exploded perspective view of a mechanism of aheight-adjustable device of the dispenser;

FIG. 9 is an exploded perspective view of a portion of theheight-adjustable device;

FIG. 10 is a vertical cross-sectional view of the dispenser;

FIG. 11 is a front view of the dispenser;

FIG. 12 is a partial vertical cross-sectional view of the dispenser,showing the state in which a bottom aperture of the dispenser is open;

FIG. 13(a) shows the state in which a flexible squeegee employed byconventional screen printing apparatus is in pressure contact with ametal screen;

FIG. 13(b) shows the state in which printed solder paste printed by theflexible squeegee is deformed;

FIG. 14(a) shows the state in which the metal squeegee employed by thepresent invention is in pressure contact with a metal screen;

FIG. 14(b) shows the state in which solder paste is properly printed onthe circuit board by the metal squeegee;

FIG. 15 shows the state in which the dispenser of the present inventionis performing printing, while the lower end of the squeegee is pressedagainst the screen with a relatively strong pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1, 2, 3, 10 and 12, there is shown a screen printingapparatus as one embodiment of the present invention. The apparatuscomprises a solder paste supply dispenser S which is arranged to bemoved along a pair of guide rails back and forth and to be raised andlowered relative to the guide rails. The dispenser S comprises abox-like housing A having a chamber 5 for containing solder paste 4 tobe printed onto a surface of a printed circuit board P through a screen1 formed of a metal plate, such as a stainless steel plate and aslit-like bottom aperture 3 formed in the bottom of the housing A andcommunicating with the chamber 5. A pair of relatively thin metal blades2A, 2B depend from the bottom of the housing A on opposite sides of thebottom aperture 3 to contact an upper surface of the screen 1 duringprinting. The screen 1 has a plurality of openings 1A therein and ispositioned on the circuit board P which is placed on a printing table 42of the apparatus.

The housing A includes a pair of side walls substantially parallel to,and spaced from, each other in a direction of travel of the dispenser Sand a front wall 7 fixedly secured to inner surfaces of forward portionsof the side walls 6A, 6B and extending perpendicular thereto. The frontwall 7 extends rearwardly and downwardly and has at the lowest portionthereof a bottom wall 7A with a rearwardly and downwardly directedportion. The housing A also includes a rear wall 8 which comprises anupper wall section 8A fixedly secured to inner sides of rearwardportions of the side walls 6A, 6B and extending perpendicular theretoand a lower flap wall section 8B having a rearwardly and downwardlydirected lower portion. As may be seen from FIG. 12, in the illustratedembodiment, the metal blade 2A as a leading blade is fixedly secured toan inner surface of rearwardly and downwardly directed portion of thebottom wall 7A of said front wall 7 and the metal blade 2B as a trailingblade serving as a squeegee blade is secured to the lower portion ofsaid lower flap wall section 8B by clamp means 8B'.

The lower flap wall section 8B is swingably and slidably connected toand depending from the fixed upper wall section 8A so that it can beswung toward and away from the front wall 7 to close and open the bottomaperture 3 which is formed between the bottom wall 7A from which theleading blade 2A depends and the lower portion of said lower flap wallsection 8B from which the squeegee blade 2B depends and, at the sametime, it can be generally vertically moved within a predetermined rangeto adjust its height.

In the illustrated embodiment, the upper wall section 8A is, as shown inFIGS. 5 through 7, composed of an upper wall member 8A1, an intermediatewall member 8A2 dismantably secured thereto and a lower wall member 8A3dismantably secured thereto. As shown in FIG. 7, the intermediate wallmember 8A2 is an elongated plate having a rectangular cross-sectionshape and serving as a connecting wall for the upper wall member 8A1 andthe lower wall member 8A3. These wall members 8A1 and 8A3 are connectedby the intermediate wall member 8A2 in such a way that an upper portionof the intermediate wall member 8A2 is frictionally fitted into a groove8A1' formed in the bottom of the upper wall member 8A1 along alongitudinal axis thereof and a lower portion of the intermediate wallmember 8A2 is also frictionally fitted into a groove 8A3' formed in thetop portion of the lower wall member 8A3 along a longitudinal axisthereof.

As may be seen from FIGS. 5 through 7 and 10, the lower flap wallsection 8B is a shaped wall block having a relatively thick body portion8Bb and a relatively thin upper portion 8Ba with a rounded portion 8Ba'which extends along a longitudinal axis of the flap wall section 8B. Onthe other hand, the lower wall member 8A3 is a generally T-shaped wallblock having a pair of flanges 8A3a, 8A3b projecting in oppositehorizontal directions. The forwardly projecting flange 8A3B is providedwith a substantially inverted U-shaped groove 12 which extends along thegroove 8A3' and which can slidably receive the rounded portion 8Ba' ofthe lower flap wall section 8B. Thus, as the flap wall section 8B isconnected to the lower wall member 8A3 of the upper wall section 8A byfitting the rounded portion 8Ba' into the inverted U-shaped groove 12,the flap wall section 8B depending therefrom can be swung toward andaway from the front wall 7.

In the illustrated embodiment, the lower flap wall section 8B isoperatively engaged with a pair of actuating means B mounted on the sidewalls 6A, 6B of housing A and for closing the aperture 3 when thedispenser S is in a non-printing position and for opening it to supply apredetermined amount of solder paste 4 contained in the chamber 5 ontothe screen 1 through the aperture 3 when the dispenser S is in aprinting position. Each of the actuating means B comprises a pneumaticactuator 20 mounted on the outer side of each of the side walls 6a, 6Bat a forward portion thereof and a link mechanism 21 which drivinglyconnects the actuator 20 to the lower flap wall section 8B through themedium of a transversely extending cylindrical member 11 to which arecessed rear portion provided on the rear side of the flap wall section8B is secured by fasteners. The cylindrical member 11 is movablysupported at its opposite end portions 11A by forwardly and downwardlyextending slots 10A, 10B respectively formed in lower portions of theside walls 6a, 6B of the housing A. The link mechanism 21 comprises abell crank 21A pivotally mounted on each side wall 6A, 6B by means of apin 21' and pivotally connected at one end to a piston rod 20A of theair cylinder 20 and a link arm 21B one end of which is pivotallyconnected to the other end of the bell crank 21A and the other end ispivoted to each of the opposite ends of the cylindrical member 11 thatprojects outwardly of the side walls 6A, 6B through the slots 10A, 10B.

Accordingly, with the actuating mechanism mentioned above, as the bellcrank 21A is pulled by the piston rod 20A when the dispenser S is atrest and the bottom aperture 3 is closed as shown in FIG. 3, thecylindrical member 11 is rearwardly pushed by the link arm 21B, therebycausing the lower flap wall section 8B to be swung away from the frontwall 7 to open the bottom aperture 3, as shown in FIG. 2. Conversely, asthe bell crank 21A is pushed by piston rod 20A when the aperture 3 isopen, the cylindrical member 11 is pulled by link arm 21B, therebycausing the flap wall section 8B to be swung toward the front wall 7 toclose the bottom aperture 3.

Indicated by reference numeral 22 is a pair of manually operableheight-adjustable devices respectively mounted on the side walls 6A, 6Bof housing A and for adjusting the height of the lower flap wall section8B having the trailing blade 2B as a squeegee blade. Each of theheight-adjustable devices 2 comprises, as shown in FIGS. 4, 8 and 9, arectangular mounting plate 24 horizontally fixed to each of the sidewalls 6A, 6B by fastening means 23, an adjusting bolt 25 verticallypiercing through the mounting plate 24 and a vertically movableadjusting plate 27 having at its upper portion a tapped hole 26 inthreaded engagement with said adjusting bolt 25. The adjusting plate 27has at an upper portion a relatively small slot 28 vertically extendingalong a vertical axis thereof and at a lower portion a relatively largeforwardly and downwardly extending slot 30 which correspondssubstantially to the slots 10A, 10B of the side walls 6A, 6B. Theopposite end portions 11A of the cylindrical member 11 projectrespectively outwardly through the slots 10A, 10B. A verticallyextending small slot 31 similar to the small slot 28 is also provided inthe adjusting plate 27 at a portion below the larger slot 30 and a pin32 fixed to each side wall 6A, 6B projects outwardly through the lowersmall slot 31. Coaxially mounted on a threaded portion of the adjustingbolt 25 between the mounting plate 24 and the adjusting plate 27 is acompression spring 33. The head 25A of adjusting bolt 25 is provided, atits peripheral surface, with scale marks 25B which make it possible forthe user to easily measure the height of the lower flap wall section 8Bby reading them and to help make fine adjustment of the height of theflap wall section 8B.

In manipulation of each of the height-adjustable devices 22, as theadjusting bolt 25 is manually rotated clockwise or counter-clockwise,the adjusting plate 27 which is in threaded engagement therewith iscaused to be displaced upwardly or downwardly and the cylindrical member11 is caused to be moved upwardly or downwardly by the adjusting plate27 for a distance corresponding generally to the amount of displacementof the adjusting plate 27 along a slope of the slot 10A, 10B in saidside wall 6A, 6B, whereby the lower flap wall section 8B secured to thecylindrical member 11 is caused to be generally vertically displacedupwards or downwards for a distance corresponding generally to theamount of displacement of the cylindrical member 11, together with thesqueegee blade 2B which is secured to the flap wall section 8B. In theillustrated embodiment, each of the height-adjustable devices 22 isdesigned so as to be capable of adjusting the height of the flap wallsection 8B within the distance of 2.0 mm.

The leading and trailing blades 2A, 2B are preferably made of relativelythin steel plate having excellent rigidity and resiliency, such ascarbon steel, alloy steel, etc. which are used as material for making aflat spring. Preferably, thickness of each of the metal blades 2A, 2B isless than 0.5 mm. However, the thickness may be within the range of 0.05mm to 0.1 mm, as long as such a steel plate as mentioned above is usedfor making the blades. In the illustrated embodiment, the lower ends2A', 2B' of the blades 2A, 2B are rounded, as shown in FIGS. 12 and 15,so that when the dispenser S is performing a printing operation,friction produced between the lower ends and the upper surface of thescreen 1 can be minimized. When a metal blade is used as a leading blade2A, it can serve as an effective cleaning blade for removing residue 4'of solder paste 4 left on the upper surface of the screen 1 during theprevious pass of the dispenser S.

Indicated by 34 is an opening which pierces through the fixed upper wallsection 8A of rear wall 8 for supplying solder paste into the chamber 5from outside of the dispenser when the amount of the paste 4 remainingin the chamber has been reduced. Indicated by 35 are openings forsupplying inert gas into the chamber 5 from the outside of dispenser Sfor the purpose of preventing the solder paste 4 in the chamber fromoxidation.

Indicated by 36 is a partition wall which divides the inner space of thehousing A, which is formed by the surrounding walls 6A, 6B, 7, 8 and thetop wall 9 mounted thereon, into the chamber 5 and a space 38 foraccommodating a device for adjusting temperature of the paste 4contained in the chamber 5. This temperature adjusting device 37comprises an electronic heater 37A and an air blower 37B for cooling thepaste 4. This blower 37B is arranged to be operated to cool the pastewhen the temperature thereof exceeds a predetermined range of degrees,such, for example, as 24° to 26° C. in response to a signal of detectionsent from a temperature sensor 37' provided in the chamber 5 fordetecting the temperature of paste 4 so that the changes in the paste 4may be held down.

The dispenser S further includes a pair of rotatable members 13, 14 inthe form of a cylindrical roller for kneading the paste 4 in the chamber5 so that the paste 4 can have a desired degree of viscosity forprinting. These members 13, 14 extend between the side walls 6A, 6B ofthe housing A and are arranged to be driven for rotation by means ofreversible motors 17, 18 supported by small bushings 15, 16 which aremounted on the inside of the side walls 6A, 6B respectively. In theillustrated embodiment, as shown in FIG. 8, rotation of each motor 17,18 is transmitted to each of the rotatable members 13, 14 through themedium of a belt drive 19 which is provided within a verticallyextending large slot 6' formed in each of the side walls 6A, 6B. Theslot 6A' is covered with a removable cover 6".

Indicated by reference numeral 39 in FIG. 10 is a deflecter member madeof a flat spring, supported by a supporting member 40 located betweenthe two rollers 13, 14 and resiliently pressed against surfaces thereofon the side facing the lower flap wall section 8B. 41 is also adeflecter member which is integrally formed with the leading metal blade2A and is resiliently pressed against a surface of the lower roller 14at a lower portion thereof.

In FIGS. 1 through 3, numeral 43 indicates a pair of supports forsupporting screen 1 thereon, 44 indicates a pair of guide rails, 45 is acarriage movably supported by the guide rails, 46 is a pneumaticcylinder mounted on the carriage 45 and for raising and lowering thedispenser S, 47 indicates a belt drive for reciprocating the carriage 45and the dispenser S along the guide rails 44 and numeral 48 indicates apair of pneumatic suspension cylinders mounted on the carriage 45 andoperatively connected to the dispenser S. These cylinders 48 serve asshock absorbers to absorb the shock to be produced when the dispenser Sis lowered and brought into contact with the screen 1 and also serve asbalancers for the dispenser S during printing.

In operation, the dispenser S is, when at rest, in an initial position,not shown and from there it is lowered and placed into a secondposition, not shown to start printing. When the dispenser S is placedinto this position, the slit-like bottom aperture 3 is opened by movingthe lower flap wall section 8B of rear wall 8 away from the front wall 7so as to be capable of supplying solder paste 4 contained in the chamber5 of the dispenser onto the upper surface of screen 1. At the same time,the rollers 13, 14 are made to rotate to knead the paste 4. In thiscase, it is preferable that the upper roller 13 is made to rotateclockwise, while the lower roller 14 is made to rotate counterclockwisein FIG. 10 so that the paste 4 can be effectively kneaded. The dispenserS is made to travel forwardly, i.e. in the direction of arrow X in FIG.2, with the lower end 2A', 2B' of the leading and trailing meal blades2A, 2B being in sliding contact with the upper surface of screen 1. Whenthe dispenser S starts printing, it is desirable that the lower roller14 is made to rotate clock-wise so that supply of the paste 4 onto thescreen 1 can be facilitated. As the squeegee blade 2B passes each of theopenings 1A in screen 1, a predetermined amount of the solder pastesupplied out of the chamber 5 is applied and urged into the openings 1Aby the squeegee blade 2B, whereby the openings 1A are filled with thepaste 4. After the squeegee blade 2B has passed all the openings 1A andreached a third position where printing is completed, the aperture 3 isclosed, rotation of each roller 13, 14 is stopped and the printedcircuit board P is downwardly moved with the printing table 42 so thatscreen 1 and the circuit board P are separated from each other andprinted deposits of solder paste having a pattern corresponding to thepattern of the openings 1A in the screen are left on metalized areas ofthe board P. After the aperture 3 has been closed, the dispenser S israised and moved rearwardly, i.e. in the direction of arrow Y in FIG. 3,to return to the initial position from said third position to completeone printing cycle.

It is desirable that after the first printing cycle has been completed,operation of the dispenser S is stopped and the operator checks on theresult of the printing performed during the first printing cycle. If theprinting effect does not meet requirements or is unsatisfactory and ifit has been found that such an undesirable result is due to the factthat the contacting pressure of the lower end 2B' of the squeegee blade2B against the upper surface of the screen 1 was inadequate orinsufficient, then the operator can easily adjust said pressure into adesirable value by manipulation of the pair of height-adjustable devices22, 22.

Referring to FIG. 13(a), there is depicted the state in which a flexiblesqueegee N employed by a conventional screen printing apparatus is inpressure contact with a metal screen 1. As shown, the lower end of thesqueegee N-projects partially into an opening 1A in the screen 1. As aresult, as shown in FIG. 13(b), the printed solder paste 4 on ametalized area of printed circuit board P is deformed such that itsupper surface is concaved.

Referring now to FIG. 14(a), there is shown the state in which the metalsqueegee 2B employed by the present invention is in pressure contactwith the metal screen 1. It will be seen from the drawing that the metalsqueegee blade 2B does not project into an opening 1A in the screen 1 atall. As a result, the thickness of the printed paste 4 is accuratelyregulated by the thickness of screen 1, as shown in FIG. 14(b).

Referring now to the drawing in FIG. 15, there is shown the state inwhich the metal squeegee blade 2A employed by the present invention ispressed against the upper surface of screen 1 with relatively strongpressure. It will be seen that the printed paste thickness is stillinvariant even under such an increased pressure. In this connection, itshould be noted that since the lower end 2B' of the squeegee blade 2B isrounded, the blade 2B can be smoothly moved across the screen 1 and alsocan protect the screen 1 from abrasion.

The surrounding walls 6A, 6B, 7, and the top wall 9 of housing A may beformed of an aluminum alloy or of synthetic resin. The partition wall 36is preferably formed of brass having excellent thermal conductivity.Both of the leading and trailing blades 2A, 2B may alternatively beformed of a hard material, such as synthetic resin having excellentmechanical strength as well as excellent chemical resistant properties,such for example, as reinforced nylon or the like having a Vickershardness of more than 10.

It is to be noted that the thickness of the printed film or deposit isdetermined principally by the thickness of the screen, while dimensionsof the same is principally determined by size of opening 1A in thescreen and that with a conventional flexible squeegee having arelatively large thickness, it is difficult, in general, to print solderpaste with high viscosity through the opening 1A of very small size, sayless than 30 micrometers in diameter.

With the dispenser S having the aforementioned height-adjustable devices22 and the aforementioned blades 2A, 2B, difficulties experienced by theprior art can be effectively overcome. According to the dispenser S ofthe present invention, the contacting pressure of the squeegee 2B to beapplied against the screen 1 can be properly adjusted, if neededdepending on the viscous material used and the thickness of the screenas used.

Further, the inside of the dispenser S can be easily cleaned by removingthe dismantleable rear wall 8 of housing A.

It is to be understood that the present invention may be embodied inother specific forms without departing from the spirit or essentialcharacteristics of the present invention. The preferred embodiments aretherefore to be considered illustrative and not restrictive. The scopeof the invention is indicated by the appended claims rather than by theforegoing descriptions and all changes or variations which fall withinthe meaning and range of the claims are intended to be embraced therein.

I claim:
 1. A screen printing apparatus for printing a viscous materialonto a surface of a board through the medium of a screen having openingstherein and an upper surface, the apparatus comprising:a viscousmaterial dispenser having a box-like housing having a chamber, aslit-like bottom aperture and a rear wall; said chamber containing saidviscous material; said slit-like bottom aperture having a leading sideand a trailing side opposed thereto; a pair of blades including aleading blade and a trailing blade, said pair of blades depending fromthe housing on substantially opposite sides of said bottom aperture tocontact said upper surface of said screen; said rear wall of saidhousing comprising a fixed upper wall section and a lower flap wallsection swingably and slidably connected thereto, depending therefromand defining the trailing side of the bottom aperture; said trailingblade serving as a squeegee blade and fixed to said flap wall section; apair of manually operable height-adjustable devices mounted on saidhousing, said flap wall being connected to said height-adjustabledevices, whereby the contacting pressure of the lower end of saidtrailing blade to be applied against said upper surface of said screencan be adjusted as desired; and a pair of actuating means mounted onsaid housing and for closing said aperture when said dispenser is in anon-printing position and for opening it to supply a predeterminedamount of the material in said chamber onto the screen through saidaperture when said dispenser is in a printing position, said flap wallsection being operatively engaged with said actuating means.
 2. A screenprinting apparatus as claimed in claim 1, wherein the viscous materialis a solder paste to be printed onto a printed circuit board to mountsemi-conductor tips and/or other electronic parts thereon.
 3. A screenprinting apparatus as claimed in claim 1, wherein:a rotatable member forkneading the viscous material is contained in the chamber.
 4. A screenprinting apparatus as claimed in claim 2, wherein the housing is aclosed type and includes another chamber for accommodating a temperatureadjusting means for adjusting a temperature of the solder pastecontained in said chamber by means of a partition wall.
 5. A screenprinting apparatus as claimed in claim 1, wherein each of the pair ofthe blades is formed of a steel strip and a lower end of each of theblades is rounded.
 6. A screen printing apparatus as claimed in claim 5,wherein a thickness of each of said blades is less than 0.5 mm.
 7. Ascreen printing apparatus as claimed in claim 1, wherein the screen isformed of a stainless steel.
 8. A screen printing apparatus as claimedin claim 1, wherein the dispenser is connected to a carriage which ismovably supported by a pair of guide rails by means of a pneumaticcylinder for raising and lowering the dispenser and a pair of pneumaticsuspension cylinders.
 9. A screen printing apparatus as claimed in claim1, wherein each of the pair of the actuating means includes a pneumaticactuator and a link mechanism which drivingly connects the actuator tothe lower flap wall section of said rear wall.
 10. A screen printingapparatus as claimed in claim 1, wherein said leading blade serves, ineach pass of the dispenser as a scraper blade for scraping off residueof the viscous material and for the removal of the residue adhered tothe upper surface of said screen during a previous pass of saiddispenser.